MOSCOW, Russia (CNN) -- Twelve people were killed and six were injured in an explosion at an open-cast mine in Russia's northwest Murmansk region, a local emergency official said Friday.

Rescue workers had only recovered four bodies by midday Friday, more than 12 hours after the explosion and resulting fire, said Irina Gretskaya, a spokeswoman for the local division of the Russia Emergency Situations Ministry.

Other bodies are in hard-to-reach areas of the mine where the temperature remained high, she said.

All the dead miners were between the ages of 20 and 37, she said.

Murmansk is near the Barents Sea and Russia's northern border with Finland.

The mining company said the explosion happened during preparations for a controlled explosion at the mine, scheduled for December 15.

More than 19 tons of explosives had already been charged and another 30 tons were being delivered to the site at the time, when an unexpected blast went off inside the cargo truck that was delivering the explosives, the Apatit Mining Company said.

The Murmansk regional prosecutor's office has opened a criminal investigation into the explosion to see whether it violated explosion and mining rules, the Interfax news agency reported, citing the prosecutor's office.

The mining company develops apatite-nepheline ores used in the production of fertilizers.